1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device having plural first transistors constituting an information memory circuit, and plural second transistors constituting a gate circuit for controlling information input and output.
2. Related Background Art
Recent development of so-called information society has stimulated the demand for memory devices, among which semiconductor memory devices occupy an important position due to their advantages performance and cost.
Among such semiconductor memory devices, there is already known a type having an information memory circuit for information storage and gate units for controlling the writing and reading of said information.
FIG. 1 is a circuit diagram showing an example of a static RAM of said type.
As shown in FIG. 1, the static RAM cell is composed of an information memory circuit 20 and gate units 21a, 21b.
The information memory circuit 20 consists of MOS transistors T1, T3 and MOS transistors T2, T4 constituting CMOS inverters.
The gate units 21a, 21b are composed of MOS transistors T5, T6 for connecting said information memory circuit 20 with read-out lines D, D, and the MOS transistors T5, T6 function as gate elements through gates thereof connected to address lines AL.
In the manufacture of a semiconductor memory having an information memory circuit and gate units such as the static RAM mentioned above, it has not been possible to overlay the information memory circuit and the gate units consisting of MOS transistors, since it is generally difficult to form a monocrystalline semiconductor layer on an insulating layer. For this reason the information memory circuit and the gate units are placed side by side on a same conductive substrate, and the reading line and the address line are placed on an insulating layer. Consequently the density of integration can only be improved by overlaying the reading line and the address line in the wiring area, and a sufficient degree of integration has therefore been difficult to achieve.